In a case of the prior art, as disclosed in the following patent document, a number of ribs are formed in vertical and radial directions in the region of a joining surface at the rear end of a cylinder block on which a transmission case is mounted.
Unexamined Patent Publication No. H10-205389
In the above-described current structure where a number of ribs are formed in vertical and radial directions, there have been problems that the weight of the whole cylinder block increases and that the casting operation of the cylinder block cannot be efficiently performed.
In views of the above-described problems of the prior art, it is an object of the present invention to provide a cylinder block that can minimize the number of ribs, thereby ensuring a lightweight construction and improving castability.
According to an aspect of the present invention, there is provided a cylinder block having bolt holes formed on the upper surface for connecting a cylinder head with bolts, arc-shaped coupling surfaces formed on the rear end surface for mounting housings of a transmission and the like, and a crankshaft bearing portion formed in the central part between the lower ends of the coupling surfaces, the improvement of the structure of the cylinder block, comprising: an arch-shaped rib joining the upper parts of the coupling surfaces into an arch-shape; right and left bolt hole bosses having bolt holes formed therein; the lower ends of the right and left bolt hole bosses extended and connected to the lower parts of the arch-shaped rib; and right and left main radial ribs respectively extended from the outside of the lower end of the right and left bolt hole bosses to the center of the crankshaft bearing portion.
According to the present invention, the upper parts of the coupling surface at the rear end of the cylinder block are joined by the arch-shaped rib into an arch-shape; the lower ends of the right and left bolt hole bosses having bolt holes formed therein extend to the lower parts of the arch-shaped rib and are joined to the arch-like rib; and the right and left main radial ribs extend from outside of the lower ends of the right and left bolt hole bosses toward the center of the crankshaft bearing portion. Therefore, when the cylinder head is coupled onto the cylinder block with bolts, although a force may act to generate a falling deformation of the bolt hole boss, the cylinder block is pulled up to the side of the cylinder head and the tightening force of the bolt is dispersed excellently in the directions of the arch-shaped rib and main radial rib. Thus, the deformations of the cylinder and crank bearing portion are minimized, so that the roundness of the crank bearing portion is secured, thereby eliminating seizure of the crank bearing portion, sealing failure and so on.
According to another aspect of the present invention, a top face lateral rib is formed in the lateral direction above the arch-shaped rib and right and left vertical ribs which connect both right and left ends of the top face lateral rib to the coupling surfaces and arch-shaped rib is formed outside the right and left bolt hole bosses.
Because the top face lateral rib is formed in the lateral direction above the arch-shaped rib and right and left vertical ribs which connect both right and left ends of the top face lateral rib to the coupling surface, and because the arch-shaped rib is formed outside the right and left bolt hole bosses, fall of the bolt hole boss due to the tightening force of the bolt can be suppressed and further, the deformation of the crank bearing portion can be suppressed excellently.
According to a further aspect of the present invention, a horizontal reinforcement rib for connecting the right and left main radial ribs is provided.
Because the horizontal reinforcement rib for connecting the right and left main radial ribs is provided, stiffness of the right and left main radial ribs is enhanced by the horizontal reinforcement rib, so that the number of the ribs on the cylinder head rear end surface can be minimized, thereby securing light weight cylinder head having an excellent castability.